a) Field of the Invention
The invention is directed to a phase-modulated interferometer for evaluating phase displacements due to changes in optical path length in the measurement arm of the interferometer. It is used in particular for precision distance measuring systems which preferably make use of the heterodyne method for evaluating.
b) Background Art
Precision distance measuring systems based on interferometers have been known since lasers were first introduced. There is a basic distinction between homodyne and heterodyne evaluating methods. Heterodyne methods are generally preferred due to the possibility of counting forward and backward and the high interpolation due to the dwindling constant or direct light component. At present, single-sideband detection is used exclusively for evaluation. Zeemann splitting or Bragg deflection are used to generate a sideband or spatially separate the sidebands. In integrated-optical heterodyne interferometers, a frequency or phase modulation can also be effected in addition to the splitting and recombination of the beam. For reasons of stability and the difficulty of forming single-mode strip waveguides on layer waveguides, and vice versa, with the aid of tapers, lenses or grids, interferometers with continuous strip waveguides are desirable. However, this excludes the acousto-optical Bragg deflection for spatial separation of the sidebands. A phase modulation can be realized in the strip waveguide on the basis of the electro-optical effect. A sideband suppression can be achieved with a precisely defined electrical controlling of the modulator. For example, in IEEE Journ. Quant. Electr. QE-18 (1982), pages 124-129, Voges et al. describe a defined electrical control of the modulator by sawtooth pulses with defined flyback and accordingly achieve a sideband suppression of 40 dB. However, production of such control signals is complex and requires a very high outlay in regulating means.